If concentration and oxidation valence state of metal ions in an aqueous solution are to be analyzed, and if an electrolyzing reaction mechanism is to be studied, or if the metal ions are to be effectively separated and purified in a continuous process, then a rapid adjustment of the oxidation state of the metal ions in the solution is required. For these purposes, some electrolytic system with large electrode area in a simple structure, which can control the oxidation state of metal ions rapidly and continuously without adding chemicals into the system, is required.
In constituting a continuous electrolyzing apparatus, the most important factor is the selection of an electrode material which is stable in the working solution, superior in its repeatability, and affording a large contact electrode area within a small space. Another important factor is the simple manufacturing of the electrode with above described electrode material.
Generally, solid metals are widely used as the electrode of an electrolyzing apparatus, because the processing of them is easy in various shapes. However, most of them are easily oxidized in the air, and therefore, the reproduceability is lowered in repeated runs, as well as being lowered in the current density.
In order to prevent these phenomena, a precise surface polishing is performed or an electrochemical pre-treatment is carried out before using the solid metal electrode. Further, these solid metal electrodes are low in the overpotential against hydrogen evolution reaction, and therefore, hydrogen is generated near the reducing potential of the concerned elements, with the result that the interpretation of the electrolytic current is interfered. Further, in manufacturing an electrolyzing apparatus with a large contact electrode area of the solid metal, the structure becomes complicated, and the size of the apparatus is inevitably increased, with the result that the manufacturing cost is increased.
On the other hand, mercury, which is a liquid metal, has the highest overpotential for hydrogen evolution reaction among metal electrodes, and therefore, it is widely used in the research on a electroredox reaction mechanism. However, it has a drawback of difficult formation of a definite shape of large area due to its property of high surface tension. Therefore, the mercury electrode is usually used in shape of a droplet or a small pool for the electrochemical analysis, so that its surface can be used as an electrode area. The polarography using the mercury droplets as a working electrode is suitable for the electroredox study owing to the mercury droplet formed freshly and regularly through a capillary tube. However, due to its structure and to the restricted electrode area, it cannot be applied to a process in which a fast adjustment of oxidation state of metal ions is required. In order to raise the overpotential of the general solid metals against hydrogen evolution reaction, there are cases in which a thin film of mercury is coated on a metal body to use it as an electrode. However, due to the instability of the coated mercury, it is impossible to use the electrode for a long time.